[dm-devel] LVM Snapshot Feature

[Bombardier, Pascal]

Hi,

In your opinion, does it make sense to be able to create non-persistent
snapshots ?
In the case of a filesystem backup, a non-persistent snapshot would be a
snapshot that we agree to lose in case of failure or reboot. If anything
happens, a server administrator would have to recreate a new snapshot
and then restart the backup.
In this case, I think metadata could be stored in memory instead of on
disk, which should improve LVM snapshot performance.

Is this theory wrong, and is this something which can be implemented
easily in the current LVM code ?

Pascal

[Luke S Crawford]

Post by Bombardier, Pascal
In your opinion, does it make sense to be able to create non-persistent
snapshots ?
In the case of a filesystem backup, a non-persistent snapshot would be a
snapshot that we agree to lose in case of failure or reboot. If anything
happens, a server administrator would have to recreate a new snapshot
and then restart the backup.

Considering the current failure mode when a snapshot becomes 'full' (that
is, when I run out of space to remap changed blocks the snapshot becomes
corrupted) this is the only way I currently feel comfortable using LVM
snapshots at all, so personally, I think snapshots that did not persist
through reboots would be quite useful.

[Adam Hamsik]

Post by Bombardier, Pascal
Hi,
In your opinion, does it make sense to be able to create non-
persistent
snapshots ?
In the case of a filesystem backup, a non-persistent snapshot would be a
snapshot that we agree to lose in case of failure or reboot. If anything
happens, a server administrator would have to recreate a new snapshot
and then restart the backup.
In this case, I think metadata could be stored in memory instead of on
disk, which should improve LVM snapshot performance.
Is this theory wrong, and is this something which can be implemented
easily in the current LVM code ?

AFAIK non-persistent snapshots are already implemented see
DM_PERSISTENT_DEV_FLAG ? I haven't look at linux kernel code how is
it done but when I solve my lvcreate -s bug on NetBSD non-persistent
snapshots are next :).

Regards

Adam.

[Mikulas Patocka]

Post by Adam Hamsik

Post by Bombardier, Pascal
Hi,
In your opinion, does it make sense to be able to create non-persistent
snapshots ?
In the case of a filesystem backup, a non-persistent snapshot would be a
snapshot that we agree to lose in case of failure or reboot. If anything
happens, a server administrator would have to recreate a new snapshot
and then restart the backup.
In this case, I think metadata could be stored in memory instead of on
disk, which should improve LVM snapshot performance.
Is this theory wrong, and is this something which can be implemented
easily in the current LVM code ?

AFAIK non-persistent snapshots are already implemented see
DM_PERSISTENT_DEV_FLAG ? I haven't look at linux kernel code how is
it done but when I solve my lvcreate -s bug on NetBSD non-persistent
snapshots are next :).
Regards
Adam.

Yes. They are implemented in device mapper in kernel --- but no userspace
lvm tool can use them. You can only create them using dmsetup.

Mikulas

[haad]

Post by Mikulas Patocka

Post by Adam Hamsik

Post by Bombardier, Pascal
Hi,
In your opinion, does it make sense to be able to create non-persistent
snapshots ?
In the case of a filesystem backup, a non-persistent snapshot would be a
snapshot that we agree to lose in case of failure or reboot. If anything
happens, a server administrator would have to recreate a new snapshot
and then restart the backup.
In this case, I think metadata could be stored in memory instead of on
disk, which should improve LVM snapshot performance.
Is this theory wrong, and is this something which can be implemented
easily in the current LVM code ?

AFAIK non-persistent snapshots are already implemented see
DM_PERSISTENT_DEV_FLAG ? I haven't look at linux kernel code how is
it done but when I solve my lvcreate -s bug on NetBSD non-persistent
snapshots are next :).
Regards
Adam.

Yes. They are implemented in device mapper in kernel --- but no userspace
lvm tool can use them. You can only create them using dmsetup.
Mikulas

Do you have list of commands which I have to run to create persistent dm
snapshot with dmsetup ? It would be handy to test my driver with dmsetup
first and only after that use lvm2tools to create persistent/non-persistent
snapshots.

Regards

Adam.